In 1980, a cross-sectional survey to measure the prevalence of dental caries and dental fluorisis was conducted in several study sites served by public water supplies that contained natural fluorides at approximately the optimum concentration recommended for maximal caries protection, and at two, three and four times the optimum. Only children who were continuos residents since birth at each site and who used the public water supply as their primary source of drinking water were included in the survey. About 800 children, ages 8-15 years, were examined. Dental caries was assessed with the DME surface index and dental fluorosis was measured traditionally with Dean's Index and with a newly developed Tooth Surface Index of Fluorosis (TSIF). Fluorosis was assessed independently in each child by each Index. In addition, color photographs of the teeth of some children were taken to depict varying degrees of fluorosis. In the spring of 1982, a group of children from four communities in Iowa with negligible concentrations of fluoride in their drinking water were examined for compararison with the children in Illinois.